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Total Likes: 5
Original Post (Thread Starter)
#860902 10/16/2021 7:56 PM
by scott garland
scott garland
while adjusting end gap on piston rings for a rebuild on my TR 6R I filed the end gap of one of the second ring (scraper ring) a little too much. The measurement should be .010--.014 I am .016 should I get another set of rings or take a chance and let this fly?
Liked Replies
#860909 Oct 16th a 09:49 PM
by Wilfred
Hi Scott. I have a Triumph bulletin dated Sept of 1971 for the B twins that the correct end gap is 12 to 18 thou measured at the bottom of the barrel. I also have a bulletin dated 25/2/70 that reads.... Since the fitting of proprietary made pistons complete with rings to our machines from engine number DU44934 on 650 models and engine number H49837 on 500cc models, the piston ring gap recommendation has been changed from the original figure of 0.010 to 0.014 in. to the revised figure of 0.015-0.020 in.

" Reference to this figure is made in Workshop Manuals both in "General Data" and in the "Engine" section "Removing and Replacing the Piston Rings, " and these references should be corrected forthwith. "

I would think that rather than being concerned about too large a gap that I would make sure that all the gaps are in the 15 to 20 thou neighbourhood. I remember reading opinions that even larger gaps are not a problem. I'm currently putting together a 500cc engine and am about to increase the 14 thou gap a little before replacing the barrel.

Hope this helps.

Cheers, Wilf
2 members like this
#860917 Oct 17th a 12:00 AM
by Tridentman
+1 with Wilf.
My first job after graduating was at the Associated Engineering Group R&D Center in Central England (in 1966!).
The AE Group included Hepworth and Grandage (Hepolite pistons) Wellworthy (pistons). Glacier Metal Bearings (plain bearings) Coventry Radiator and Presswork (radiators and heat exchangers) and various other engineering companies in the automotive and aerospace industries.
I worked primarily on heat exchanger projects but I shared an office with another engineer who worked on pistons and piston rings.
Naturally we talked about each others work.
Where is this getting to?
Well--my colleague did a project to see how big the piston ring gaps could be before affecting blow by and power.
Test runs were done on a single cylinder engine of about 500cc.
Separate runs were done with piston ring gaps of the recommended minimum up to 1/8"
The power was measured as was piston ring blow by.
Over the range of piston ring gaps used there was no measurable difference in power or piston ring blow by.

So I second Wilfs recommendation in going for 20 thou.
The gap can be too small but not too big (within reason of course).
1 member likes this
#861196 Oct 19th a 09:37 PM
by The Bonneville Shop
The Bonneville Shop
This topic has come up recently on some of the Facebook forums. I first read the Triumph service bulletin recommending the widening of piston ring end gaps about 12-13 years ago and began aiming for .016-.018" end gaps. Never had any trouble when I was working on customer bikes and still continue this practice. I think you'll be fine, Scott.
1 member likes this
#861584 Oct 24th a 06:04 PM
by TR7RVMan
Hi, Interesting read. Andy & I at the Porsche dealership started tracking ring gap position over many hundreds of pistons on disassembly. The rings are indeed in constant motion. That is well known.

Wider gap being not a problem is pretty commonly known.

The side clearance of rings is important, more important on asymmetrical ring profiles. Also effects the time it takes for ring to move tightly to bottom of groove for best pressure seal.

To this day every ring & motor manufacturer states stagger rings. Certainly moments after start up they will rotate. The piston rocks when cold on the thrust face. New ring gap can be not perfect fit to bore, meaning tips of ring at gap. With this edge on thrust face on initial start up scratching bore can occur. Notice can, not will occur. Initial break in happens in a few seconds after start up. A very important first 2-3 seconds to long life & low oil consumption. Full face contact of rings takes a little longer.

Andy & I always followed best practices. There is a reason best practices are known as best practices. They give consistent good results. They just do.

Not all the techs followed best practices, unfortunately for that customer. I tracked this as well. Every single last one of these guys had a pattern of failures & comebacks. Most the time the motor/rings was good enough so customers accepted it. Overall motor life was about half as long as the best practices motors. I would have discharged these techs, but management only looks at dollars short term. Fact of life.

I visited Mercedes factory 2003 on very special tour of engine plant. We spoke to the techs as most could speak English. It was important to them to line gaps in 3rds off thrust face, rings, pistons, bores well oiled with Mercedes running in oil. Very, very few failures.

Pistons cool a great deal through rings to cylinder wall. Full face contact, ring to wall, ring side to groove is a must for good operation. Careful assembly, break in oil, a at least reasonable running in, it just works.

Ring expanders on oil rings is still commonly used. A matter of opinion which is best. Both have good results.

What I find amazing is you pull head on Japanese car. Bore has .030+” taper. 200000+ miles on original rings. Pistons rattle like mad. It runs perfectly & uses less than a US qt of oil in 3k miles. That is a study in rings!
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